The surface of the tin-plated layer of an electrolytic tin-plated steel strip as plated by the conventional continuous electrolytic tin-plating method is mat and has no gloss. In order to brighten the surface of the thus tin-plated layer, it is a usual practice to heat said strip in a heating furnace for rapidly fusing the tin-plated layer thereof, and then, immediately, to quench said strip in a quenching tank to cause rapid solidification of said fused tin-plated layer.
If the quenching is improperly applied, however, dirt patterns which look like dried stains of dirty water (hereinafter referred to as "quench stain") are produced on the surface of the tin-plated layer of said strip, considerably reducing the commercial value of the strip. Said quench stains are produced by an unevenly quenched tin-plated layer due to an irregular quenching rate caused by the non-uniform contact between the strip and the quenching liquid, which is brought about by surface turbulence of the quenching liquid in the quenching tank on introducing the strip into it, splashes of quenching liquid onto the strip, and uneven deposit of a steam film, which is generated by quenching, on the strip.
With a view to preventing the production of the above-mentioned quench stains, there have been a number of proposals. In a method disclosed in U.S. Pat. No. 3,358,980, for instance, two compartments with a hood and space plates are installed in a quenching tank, and surface turbulence on the surface of quenching liquid in the quenching tank caused by the incoming strip is prevented with the use of said spaced plates. Besides, a narrow region is confined by said spaced plates. The heated strip is directed into this narrow region. The quenching liquid first fed into said compartments, and after flowing under the lower end of a spaced plate and over the upper end of the other spaced plate, flows down into the quenching tank by gravity along both surfaces of the strip, in the same direction as the travel of the strip, and almost in parallel with the strip, in said narrow region, and after filling up the quenching tank, the quenching liquid overflows. Initially, the strip, brought into contact with the quenching liquid in said narrow region, is quenched at a relatively slow quenching rate, and secondly is moved into the quenching tank and quenched down to a prescribed temperature. Further, a temperature sensing device is provided in said narrow region to control the quenching liquid temperature.
According to the above-mentioned method, no surface turbulence is produced on the surface of the quenching liquid in the quenching tank on introducing the strip into it, so that the tin-plated layer is quenched uniformly, permitting prevention of quench stains. However, in this method, because the quenching liquid comes into contact with the strip while flowing down in said narrow region by gravity, the impact of the quenching liquid against the strip is small. Nevertheless, with the recent speeding-up of a continuous electrolytic tin-plating line, the moving speed of strip has been accelerated up to some 300 - 450 m/min. Accordingly, quenching capacity obtained only by a stream of quenching liquid by gravity as is the case of this method is insufficient. With an insufficient quenching capacity, steam generated at the interface between the strip and the quenching liquid accompanies the strip, being deposited on the surface of the strip. As a result, irregularity is found in the quenching rate of the tin-plated layer, which is not uniformly quenched, so that it is impossible to completely prevent the occurrence of quench stains. In other words, this method is not applicable to a high-speed continuous electrolytic tin-plating line. Besides, it is necessary to provide the quenching tank with a hood, spaced plates and compartments, leading to increased costs of quenching facilities.
Further, in the method disclosed in U.S. Pat. No. 3,410,734, an elongated conduit section of rectangular cross-section which provides a restricted quench channel extends upwardly from a quenching tank. A quenching liquid supplied into the quenching tank, after filling up the quenching tank, comes up in said restricted quench channel and flows over its upper end into a trough. Closely adjacent the upper end of said restricted quench channel, a plurality of submerged jet or spray units are provided for directing streams of quenching liquid toward the strip across the entire width thereof. A heated steel strip moves vertically downward from a heating furnace and enters the restricted quench channel where it is immediately immersed in the upwardly flowing stream of quenching liquid. In addition, the submerged jet or spray units direct streams of quenching liquid against the strip in a direction generally normal to the strip. Said submerged jet or spray units use a large quantity of quenching liquid with a relatively low pressure of about 1.4 - 2.1 kg/cm.sup.2.
According to the above-mentioned method, the strip can be quenched over the entire width thereof uniformly and at a high rate. Therefore, this method is applicable, in particular, for obtaining strips having martensitic microstructure and superior in flatness. However, this method has no special regard for the prevention of quench stains. The method has another disadvantage of requiring a complicated quenching device which increases the installation costs.
There is also known a method which comprises fusing the tin-plated layer of a continuously electrolytic tin-plated steel strip in a heating furnace, for the purpose of improving the corrosion resistance on either the upper or under surface of said strip, then spraying quenching liquid on either the upper or under surface of said strip, in the air or below the surface of the quenching liquid in a quenching tank, thus quenching slowly while holding a temperature differential between both surfaces of said strip, to coarsen the grain size of tin on one surface of said strip (refer to the Japanese Patent Publication No. 6.641/67).
In this method, in which the quenching liquid is sprayed only on one surface of the strip, the quenching capacity is insufficient, and no regard is given to uniform quenching of the strip over the entire width thereof. That is, in this method, increasing the pressure or the volume of water of submerged spray in the quenching tank in an attempt to improve the quenching capacity brings more serious surface turbulence of quenching liquid in the quenching tank without permitting uniform quenching of the strip over the entire width thereof, resulting in the impossibility of preventing the production of quench stains. Moreover, quenching in the air by this method aims merely at controlling tin crystal, with no regard to uniform quenching of the strip over the entire width thereof.
As mentioned above, in view of the fact that the conventional methods for quenching a continuously electrolytic tin-plated steel strip and for preventing quench stains thereon are unable to prevent the occurrence of quench stains in a satisfactory manner, there have been proposed the following two methods:
1. A method for continuously quenching a continuously electrolytic tin-plated steel strip in moving, which comprises applying a first-step quenching at a relatively low rate to spray streams of quenching liquid mist onto a continuously electrolytic tin-plated steel strip with the tin-plated layer thereof fused by heating, in the air above the surface of the quenching liquid in a quenching tank, and immediately after said first-step quenching, applying a second-step quenching at a relatively high rate to spray high-pressure and large quantity streams of quenching liquid onto the surface of said strip below the surface of said quenching liquid in said quenching tank, thereby preventing the occurrence of quench stains on the surface of the tin-plated layer of said strip (refer to the Japanese Patent Provisional Publication No. 74,532/75); and
2. A method for continuously quenching a continuously electrolytic tin-plated steel strip in moving, which comprises applying a first-step quenching at a relatively low rate to spray high-pressure streams of quenching liquid along a guide plate onto a continuously electrolytic tin-plated steel strip with the tin-plated layer thereof fused by heating, in the air above the surface quenching liquid in a quenching tank, and immediately after said first-step quenching, applying a second-step quenching at a relatively high rate to spray high-pressure and large quantity streams of quenching liquid onto the surface of said strip below the surface of the quenching liquid in said quenching tank, thereby preventing the occurrence of quench stains on the surface of the tin-plated layer of said strip (refer to the Japanese Patent Provisional Publication No. 75,131/76).
According to the above-mentioned methods (1) and (2) it is possible to largely minimize the occurrence of quench stains on causing rapid fusion and rapid solidification of the tin-plated layer of a continuously electrolytic tin-plated steel strip. In said method (1), however, in which a quenching liquid mist is sprayed onto the strip in the first-step quenching, the large amount of heat contained in the strip heated in a heating furnace may lead to a shortage in the quenching capacity.
Furthermore, in said methods (1) and (2), streams of quenching liquid are sprayed onto the strip, using a plurality of spray nozzles provided over the entire strip width symmetrically on the both sides of said strip in moving substantially vertically downward and on substantially the same horizontal level. This makes it difficult to maintain a constant pressure and a constant flow rate of the sprayed quenching liquid in the strip width direction under the effect of the mutual interference between the streams of quenching liquid sprayed from different spray nozzles located adjacent to each other. More specifically, each stream of quenching liquid sprayed from a spray nozzle diverges more at a longer distance from the nozzle hole, so that the distribution of quenching liquid becomes non-uniform between the center and the peripheral portions of each stream. This non-uniformity of the quenching liquid distribution for each spray nozzle is more serious accordingly as the quenching liquid pressure becomes higher. Even if it is attempted to keep a uniform quenching liquid distribution for each spray nozzle in the strip width direction by adjusting the number of spray nozzles and/or the intervals between spray nozzles, a change in the quenching liquid pressure leads to a corresponding change in the overlapping pattern of adjacent streams of quenching liquid, and hence to a local change in the amount of sprayed quenching liquid in the strip width direction. In said methods (1) and (2) mentioned above, therefore, it is not always easy to quench a strip uniformly over the entire width thereof on substantially the same horizontal level.
In view of the foregoing, although said methods (1) and (2) are successful in largely reducing quench stains occurring on the rapid fusion and the rapid solidification of the tin-plated layer of a continuously electrolytic tin-plated steel strip, sufficiently satisfactory effects have not as yet been obtained.